1. Field of the Invention
The present invention relates to planar magnetron sputtering, and more specifically to a cooling apparatus and method for use in planar magnetron sputtering equipment.
2. Description of Related Art
Sputtering describes a number of physical techniques commonly used in, for example, the semiconductor industry for the deposition of thin films of aluminum and aluminum alloys, refractory metal silicides, gold, copper, titanium-tungsten, tungsten, molybdenum, and less commonly silicon dioxide and silicon on an item, for example a wafer being processed. In general, the techniques involve producing a gas plasma of ionized inert gas "particles" (atoms or molecules) by using an electrical field in an evacuated chamber. The ionized particles are then directed toward a "target" and collide with it. As a result of the collisions, free atoms of the target material are released from the surface of the target, essentially converting the target material to its gas phase. Most of the free atoms which escape the target surface condense and form (deposit) a thin film on the surface of the wafer being processed, which is located a relatively short distance from the target.
One common sputtering technique is magnetron sputtering. When processing wafers using magnetron sputtering, a magnetic field is used to concentrate sputtering action in the region of the magnetic field so that sputtering occurs at a higher rate and at a lower process pressure. The target itself is electrically biased with respect to the wafer and chamber, and functions as a cathode. Objectives in engineering the cathode and its associated magnetic field source include uniform erosion of the target and uniform deposition of pure target material on the wafer being processed.
During sputtering, if magnets generating the magnetic field are stationary at a location, then continuous sputtering consumes the sputtering target thickness at that location quickly and generates hot spots at the locations of sputtering. To avoid contaminating the process, sputtering is stopped before the non-uniform wear pattern has consumed the full thickness of the target material at any point. If any point on the target plate behind the target were to be reached, sputtering of the target plate material (usually copper) would occur, contaminating the vacuum chamber and the wafer being processed with copper. Because of the non-uniform pattern of target utilization, the sputtering must be stopped when a large percentage of the target still remains.
To attempt to increase the target utilization and prolong the target's life, magnets in a magnet housing, have been moved in an oscillatory manner using various techniques in various motions. These techniques result in increasing the target utilization over that provided by stationary magnets, but also result in grooves, racetracks, or other non-uniform wear patterns being formed in the target as the target is consumed. The non-uniform target utilization takes place because the magnets and their associated magnetic field, as they are configured and moved, do not dwell uniformly over the target to be sputtered, resulting in a low percentage target utilization, as well as a non-uniform heat build-up directly associated with the non-uniform target material utilization pattern.
One approach involving the cathode in a fixed magnet magnetron sputtering apparatus is described in U.S. Pat. No. 4,680,061, issued July 14, 1987, to Lamont, Jr., and U.S. Pat. No. 4,100,055, issued July 11, 1978 to Rainey. The target is shaped like a ring, which is said to provide good deposition uniformity without using relative movement between the source and the wafer, as had previously been done.
Another approach involving a magnetic field source in a planar magnetron sputtering apparatus is described in U.S. Pat. No. 4,444,643, issued Apr. 1151 24, 1984, to Garrett. In contrast to fixed magnetron sputtering apparatus, the Garrett apparatus moves a magnetic field source across the non-vacuum side of the target to sweep magnetic flux lines over the target surface. Since maximum erosion of a target occurs where lines of magnetic flux are parallel with the surface of the target, the sweeping is said to avoid the "racetrack" grooves found in prior sputtering devices having fixed magnetic field sources, and thereby provides greater uniformity of target erosion. The magnetic field source of Garrett's apparatus includes a magnet and ring subassembly, the magnetically permeable ring thereof having a plurality of permanent magnets arranged radially inward from the inner circumference of the ring and in paired symmetry. The axial arrangement of the magnets is said to cause the creation of a series of loop pairs of flux in planes normal to the plane of the target and long diameters through the paired magnets.
Another swept field approach is described in U.S. Pat. No. 4,714,536, issued Dec. 22, 1987, to Freeman et al. The magnet assembly of Freeman et al. is rotated about a central axis relative to the target surface and simultaneously rotated about a second axis spaced from the central axis, with the magnet assembly being mounted off-center with respect to the second axis. The resulting pattern is said to be essentially an epicycloid, which is displaced about the axis of rotation with each successive revolution. The particular path traced by the magnet assembly is dependent upon the radii and gear ratios of the driving motor assembly. The magnet assembly itself contains permanent magnets mounted with their north-south axis aligned with radii of the cup-shaped holder, such that the north poles of each of the magnets are adjacent the center of the holder.
Conventional cooling methods in prior art sputtering devices employ a cavity behind or internal to a plate being heated by sputtering; see, e.g., the patents to Lamont, Jr., Rainey, Garrett, and Freeman mentioned above, as well as U.S. Pat. Nos. 3,956,093 issued on May, 11, 1976 to McLeod; 4,116,806 issued on Sep. 26, 1978 to Love et al.; and 4,175,030 issued on Nov. 20, 1979 to Love et al. In these patents, water or other cooling liquid is provided through an opening to fixed internal passages of the device or a cavity of the device. A separate opening allows the water to be discharged from the device. The devices having fixed internal passages route the cooling water through these passages, which are adjacent to the heat generated elements of the device, to cool them. In the devices where the water is routed to a cavity, the direction of the flow of water as well as the movement of a mechanism inside the cavity contribute to the agitation of the cooling liquid as it cools the heat generating elements before the cooling liquid exits the cavity. These prior art methods for cooling devices having internal cavities for cooling sometimes create large and often random differences in actual water flow and in the temperature of a heat generated target plate from one side to the other and across the plate. The water cavity is pressurized as coolant flows. Pressurizing the water cavity in contact with the target plate requires that it be strengthened, because the pressure within the water chamber when added to the vacuum pressure in the evacuated sputtering cavity provides a large differential pressure across the target plate. The target plate must be sized to support this large differential pressure.
While cooling has also been proposed by drilling gun bore type channels in the target backing plate similar to the fixed cooling passages in the prior art, the location of these channels necessarily causes localized hot spots between the channels and in the prior art temperature increases away from the location of cooling liquid flow. Also, drilling, locating, and connecting cooling liquid piping to these channels unnecessarily complicates the construction of the target backing plate.
The distance between the magnets and the surface of the target affects the degree to which sputtering is concentrated by the magnet's magnetic field. For a given magnet design, a short distance causes sputtering to be more highly concentrated than a greater distance. The highest sputtering concentration occurs if a target having a negligible thickness is mounted directly on the magnets. Every additional increment of distance between the surface of the target and the magnets reduces the influence of the magnetic field on sputtering of the target.
A thick target backing plate is necessary in a pressurized water chamber device to attain the additional strength needed to support the pressure behind the target backing plate, and is also necessary in a device using a target backing plate which has been drilled for cooling and therefore requires additional thickness to retain its nominal strength. Unfortunately, the use of a thick target backing plate, which together with the target forms a target plate assembly, adds extra thickness to the target plate assembly. To avoid a change in magnetic flux at the target's surface, a thin target must be substituted to maintain the same distance between the magnets and the target's surface. Alternately, a reduced magnetic flux at the target surface, as a result of increased distance (thickness) between the magnets and the surface of the target, must be accepted. Therefore, the need to increase the thickness of the target backing plate to provide more strength is in conflict with the desire to reduce the distance between the surface of the target and the magnets to concentrate the location of sputtering.
Despite considerable improvement in the engineering of targets and magnetic field sources for planar magnetron sputtering equipment, uniform erosion of the target and uniform deposition of pure target materials on the wafer remain problematic, particularly where sputtered material is deposited on large diameter wafers.